Project Summary/Abstract Inherited retinal degeneration in humans is an incurable disease that causes blindness. A variety of genes and mutations in both coding and non-coding regions have been associated with retinal degeneration but little is known about the molecular mechanism of disease. This proposal focuses on the highly-conserved enzyme IMP dehydrogenase 1 (IMPDH1), a key regulatory enzyme in purine biosynthesis. Nine different missense mutations in IMPDH1 have been identified in humans that lead to retinal degeneration in the form of autosomal dominant retinitis pigmentosa, with varying degrees of severity and age of onset. None of the mutations has a direct effect on enzyme activity or results in a disease phenotype in any tissue other than the retina. The lack of direct enzymatic affect or an animal model to study IMPDH1 retinal degeneration has stalled progress towards determining the mechanism of disease. IMPDH forms microns-long dynamic filamentous structures in vitro and in vivo in response to changing metabolic conditions. The physiological conditions that promote assembly/disassembly and cellular function of the filaments remain unclear. I have discovered that IMPDH1 retinal disease mutations have a direct effect on the ability of the enzyme to polymerize, either preventing polymerization entirely or promoting hyper-assembly. Here, I propose a collaborative and multidisciplinary approach to combine high resolution cryo-EM with the development of the first animal model to study IMPDH1-associated retinal degeneration. This effort will provide novel insight into the role of IMPDH1 in photoreceptors and lay the groundwork for future therapies to prevent blindness due to cell death. This varied and ambitious proposal builds off my own training in cryo-EM and provides the opportunity for me to gain experience working with a model organism and utilizing different imaging techniques. In addition to becoming an expert cryo-EM user and gaining experience working with a model organism, my fellowship training plan focuses on seeking out opportunities to present this research and hone my science communication skills. The experience gained from the proposal will set me on the track for a productive postdoc and ultimately running my own independent lab. Hypothesis: IMPDH1 retinal degeneration mutations affect the ability of the enzyme to form ultrastructures which leads to a dysregulation of the enzyme, resulting in disruption of nucleotide homeostasis and eventually cell death. Aim 1: Determine IMPDH1 filament structure and function in vitro and localization in the retina. Aim 2: Define biochemical and physiological consequences of IMPDH1 retinopathy mutants